Our invention relates to cathode ray tubes (CRTs) such as those used for oscilloscopic and storage applications. More specifically, our invention pertains to improvements in or relating to CRTs of the type having a plurality of quadrupolar lenses arranged in a row along the tube axis as parts of the electron gun, the improvements being designed for easy refocusing of the beam at the screen or target.
It has been known to incorporate three quadrupolar lenses in the electron gun of CRTs and to connect them to separate variable potential sources. European patent application Publication No. 241,945 and Japanese Laid Open patent application No. 59-134531 are examples of prior art teaching such quadrupolar lenses. Supplied with proper operating potentials, the quadrupolar lenses conjointly serve to create a beam spot on the target with a minimum of out-of-roundness. A problem has existed, however, with this known type of CRT in connection with the focusing of the electron beam at the target, as discussed in more detail hereafter.
The cathode of the electron gun emits a beam of electrons following different paths depending upon a potential impressed to the control electrode. Varying the potential on the control electrode alters the paths of the electrons and thus changes the position in which they converge or cross over on the tube axis. In the prior art CRTs having three quadrupolar lenses, such shifting of the crossover point on the tube axis has required readjustment of as many as three different potentials, or even six different potentials consisting of three different positive potentials and three different negative potentials, on the constituent electrodes of the quadrupolar lenses in order to refocus the beam at the target.